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National Health Interview Survey (NHIS) Data

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Background Information

The National Health Interview Survey (NHIS) is a multistage probability sample survey conducted annually by the National Center for Health Statistics (NCHS), Centers for Disease Control and Prevention (CDC). The NHIS administers face-to-face interviews in a nationally representative sample of households. Each week personnel of the U.S. Department of Commerce, Bureau of the Census interview a probability sample of the civilian noninstitutionalized population of the United States. Bureau of the Census personnel obtain information about the health and other characteristics of each member of the household. The NHIS is the principal source of national asthma prevalence data for the United States.

The NHIS core questionnaire items, which solicit health and demographic information about the population, are revised every 10–15 years. The last major revision occurred in 1997. The redesigned NHIS, introduced in 1997, consists of a Basic Module or Core and variable Supplements. The Basic Module, which remains largely unchanged from year to year, consists of three components: the Family Core, the Sample Adult Core, and the Sample Child Core. The Family Core component collects information about everyone in the family, including information about household composition, sociodemographic characteristics, activity limitations, injuries, health insurance coverage, and access to and use of health care services. From each family in the NHIS, one sample adult and one sample child (if any children under age 18 years are present) are randomly selected. The Sample Adult Core and the Sample Child Core questionnaires collect basic information about health status, health care services, and behavior from the sample adult and from an adult proxy for the sample child.

Since 1997, the NHIS has gathered information about lifetime asthma and asthma attacks or episodes from the Sample Adult Core and Sample Child Core questionnaires. A positive response to the question: "Has a doctor or other health professional ever told you that you had asthma?" determined lifetime asthma. To determine the prevalence of asthma episodes or attacks, persons answering "yes" to the lifetime asthma question were then asked, "During the past 12 months, have you had an episode of asthma or an asthma attack?" Since 2001, current asthma status has also been collected. To determine current asthma, persons answering "yes" to the lifetime asthma question were then asked, "Do you still have asthma?"

The prevalence measures of lifetime asthma, current asthma, and having one or more asthma attacks among those with current asthma have been constructed from the most recent NHIS data. Responses to these questions have been tabulated for various demographic groups.

The asthma prevalence measures calculated from NHIS data are weighted to provide national asthma estimates. The Sample Adult Weight (Final Annual) and Sample Child Weights (Final Annual) are used to obtain the estimates. The weights are assigned according to NHIS methodology, with weighting information found in the "NHIS Survey Description" documents.

Survey respondents’ race is categorized using a "bridge" race variable available on the NHIS public use files that allows comparisons of recent NHIS data with data from previous years. This variable was used to help maintain historic trends of national asthma prevalence estimates dating back to the 1980s.

Counts for persons of unknown asthma status are not included in the calculation of the national estimates. Only respondents answering "yes" or "no" to the survey questions are included. Responses of "don’t know" and "refused" were excluded.

Because NHIS data are based on a sample of the population, they are subject to sampling error. Standard error is a measure of sampling error. It measures the variation that might occur by chance because only a sample of the population is surveyed. The chances are about 95 in 100 that an estimate from the sample would differ from the total population by no more than twice the standard error. Standard errors are reported in the attached tables in order to indicate the reliability of the calculated national estimates. Estimates and standard errors were calculated using SUDAAN Release 7.5 or later. † The Taylor series linearization method was used for variance estimation in SUDAAN.

As an indicator of the reliability of the estimates, the relative standard error (RSE) of all estimates was also calculated. The RSE is defined as the standard error of the estimate divided by the estimate itself. Estimates with RSEs from 30% through 50% are considered statistically unreliable and are indicated with an asterisk (*). Estimates with RSEs more than 50% were suppressed, with estimates replaced by (**).

Information (population estimates, prevalence percent, and standard error) for lifetime asthma, current asthma, and one or more asthma attacks (in the past 12 months) among those with current asthma are presented for the United States in the following tables.